In a host computer, an application program such as a word processor or a spreadsheet operates on a basic software program such as Windows (trademark of Microsoft Corporation). In a case where printing is performed by using a printing apparatus, such application program calls a function of graphic subsystem among subsystems provided by the basic software program.
For example, the graphic subsystem, which is called a “GDI” (Graphic Device Interface) in the Windows system, handles processing on image information for display and/or a printer. To absorb the difference in functions and characteristics of the respective devices such as the display and the printer, the GDI dynamically links up with a module called a device driver, to perform output processing to the respective devices.
In a module for the printer, called a printer driver, is provided with a group of functions called a “DDI” (Device Driver Interface), to be pre-installed in the device driver in correspondence with the capability, functions and the like of the printer. The DDI function group is arbitrarily called from the GDI by converting an API (Application Programming Interface) call by the application program into data for the device driver. In this manner, a print request from the application program is sequentially processed by the GDI via the printer driver.
When such application of the host computer outputs an image to the printing apparatus, the image is outputted from the printer driver in RGB representation for representing colors by using additive primary colors red (R), green (G) and blue (B), or YMCK representation for representing colors by using yellow (Y), magenta (M), cyan (C) and black (K) in image formation based on the subtractive processing principle.
FIG. 1 is a schematic view for briefly explaining the processing in the case of RGB representation in the host computer.
In FIG. 1, a rectangular object D1 is filled with a red grid pattern, an elliptic object D2 is filled with yellow, a rectangular object D3 is filled with a green hatching pattern, and a circular object D4, filled with a blue tile pattern. When these 4 objects are processed, the printer driver generally performs processing on the objects D1, D2, D3 and D4, in the order of delivery from the system. At this time, ROP (Raster OPeration) characteristic of the GDI of the Windows system exists as rendering information of the objects delivered from the system.
The ROP, which is rendering processing to perform logical operation between images, is used for designation of logical rendering of object, and is provided as ROP2, ROP3 and ROP4 in correspondence with the number of the objects. The ROP designates the shape of a object, a filling pattern, and a logical operation determined mutually with respect to a destination of rendering.
The rendering of the objects D1 to D3 must be sequentially performed since in some cases, a destination as a result of immediately previous rendering is referred to in accordance with designation by the ROP. That is, unless the status of the destination as a result of rendering of the objects D1 to D3 is obtained, a proper result cannot be obtained from rendering of the object D4 based on the ROP.
Generally, these rendering processings are premised on monochrome image or processing in RGB color space.
The printing apparatus such as a printer, which inputs data from the host computer and performs printing, performs image processing in YMCK color space. For this purpose, as shown in FIG. 2, when data in mixed RGB representation and YMCK representation is inputted from the host computer, the printing apparatus converts the input RGB representation data and ROP code on the premise of RGB representation into YMCK representation data, and generates a bit image by a YMCK renderer (rendering generation device).
In a case where RGB representation data is inputted into the printing apparatus and converted to YMCK representation data, the color reproducibility of the object D4 which is not overlapped with the other objects, as shown in FIG. 1, can be approximately maintained by the performance of PDL (Page Description Language) However, regarding the objects D1 to D3 where regions S1 and S2 overlap with each other (in the case of duplication or transparency), in some cases, the validity of the image cannot be maintained even though the printing apparatus processes the above-described ROP code. This problem frequently occurs as the number of duplication is large.
Accordingly, upon rendering of object converted from RGB representation to YMCK representation, the image generating device in the printing apparatus re-converts an object having a duplication region to RGB representation again, then performs RGB rendering, then compresses the bit image and holds it, and transmits the data to a printer engine at necessary printing timing. This processing increases the load on the system and degrades the throughput of the printing apparatus.